1. Field of the Invention
The present invention relates to systems in which different modules accept different voltage levels, and more particularly, to a voltage translator that includes gate oxide breakdown protection.
2. Discussion of the Related Art
Many modern systems combine modules accepting different voltage levels. For example, in a laptop computer, processor modules powered by a 3.3 V voltage are combined with disk drives powered by a 5.0 V voltage. In order to translate the signal produced by a 3.3 V module to a level acceptable by a 5.0 V module, a simple CMOS inverter powered by 5.0 V can be used. However, such a configuration consumes excessive power during normal operation and can also allow a leakage path to form between the signal bus and the 3.3 V power supply when the bus is driven by the 5.0 V module. U.S. Pat. Nos. 5,406,140 and 5,534,795, issued Apr. 11, 1995 and Jul. 9, 1996, respectively, to Wert et al., describe a low power-consumption voltage translator providing protection against such reverse charge leakage when a voltage on a bus is larger than a power supply voltage of a module connected to the bus. To provide an interface between a module powered by a first power supply voltage and a second module powered by a larger second power supply voltage, the voltage translator of Wert et al. requires the use of both power supply voltages. The maximum allowable voltage drop across a gate oxide in the circuit is limited by the devices using first power supply voltage. However, in the invention of Wert et al., a maximum voltage drop equal to the second power supply voltage can be created across the gate oxide of a device in the voltage translator, which can lead to gate oxide breakdown.
Accordingly, it is desirable to provide an efficient voltage translator that prevents reverse charge leakage while limiting the maximum voltage that appears across its gate oxide.